Prior art systems are known in which a pacer and defibrillator are combined in a single unit for pacing the heart and for defibrillating the heart when required. Pacers provide, to the heart, relatively low energy pacing pulses while defibrillators, also known as cardioverters, provide far greater energy. An example of an implantable cardiac/defibrillator is found in Rubin U.S. Pat. No. 3,857,398.
We have found that it is desirable to provide dual chamber bradycardia support to defibrillator patients. We have also discovered a method for preventing the incidence of atrial fibrillation caused by ventricular defibrillation shocks.
In prior art implantable defibrillators, none are known with dual chamber pacing and defibrillation modalities. Further, prior art defibrillators are typically synchronous to the ventricle.
We have found that a defibrillator that has dual chamber pacing capability can be provided, with the preferred mode being a DDI.sup.1 pacer. Although DDD pacing is typically chosen by cardiologists when available, we have found that DDD pacing is inappropriate for defibrillators. That is because the ability to track the ventricular pacing rate in the presence of an atrial arrhythmia can be arrhythmogenic. Further, DVI pacing is asynchronous in the atrium, and is likely to be arrhythmogenic in those patients in whom a propensity for arrhythmias is known. In contrast, DDI pacing neither tracks nor is asynchronous and therefore we consider DDI pacing to be the preferred mode. FNT .sup.1 The symbols DDI, DDD, DVI, etc. are used herein in accordance with the Intersociety Commission on Heart Diseases Resources code.
We have also found that with a sensing electrode in the atrium as well as in the ventricle, it is possible to synchronize the shock to either chamber. Generally if there is a fast ventricular tachycardia or fibrillation, the synchrony with the ventricle is unimportant and it is better to synchronize to the atrium to avoid inducing atrial fibrillation. However, for a slow ventricular tachycardia, it may still be desirable to synchronize with the ventricle in order to avoid inducing ventricular fibrillation. In this case it may still be possible to synchronize with the "safe zone" of the atrial cycle, that is, within either approximately 80 msec. of the P-wave or 230 msec. after it.
It is, therefore, an object of the present invention to provide dual chamber bradycardia support for patients with the need for implantable defibrillation, with the minimum of adverse device mode interaction.
Another object of the present invention is to reduce the incidence of atrial fibrillation induced by shocks for ventricular tachyarrhythmias.
Other objects and advantages of the present invention will become apparent as the description proceeds.